Frictional engagement element and automatic transmission

ABSTRACT

Provided is a frictional engagement element including: a plurality of friction plates; a piston movable between a releasing position and an engaging position; and a first urging mechanism and a second urging mechanism configured to urge the piston in an engaging direction from the releasing position toward the engaging position. Urging forces of both of the first urging mechanism and the second urging mechanism act on the piston from the releasing position to a first position in the engaging direction. The urging force of the first urging mechanism acts on the piston from the first position to a second position closer to the engaging position than the first position. The first position is a predetermined position where the piston does not abut against the friction plates, and the second position is a position where the piston abuts against the friction plates and the clearances are reduced.

TECHNICAL FIELD

The present invention relates to a frictional engagement elementprovided with a plurality of friction plates and a piston, and anautomatic transmission using the frictional engagement element.

BACKGROUND ART

Automatic transmissions to be mounted on vehicles such as automobilesinclude planetary gear sets and a plurality of frictional engagementelements such as multiplate clutches and multiplate brakes. Theplurality of frictional engagement elements are selectively engageddepending on an operation state of an engine so that gears areautomatically shifted to predetermined gear positions. The frictionalengagement element includes a plurality of friction plates arranged withclearances therebetween, and a piston configured to press the frictionplates. The piston moves between an engaging position where the frictionplates are brought into an engaged state by pressing the frictionplates, and a releasing position where the friction plates are broughtinto a released state by releasing the pressing.

Japanese Unexamined Patent Publication No. 2001-65607 discloses africtional engagement element to which a spring member configured tourge the piston toward the engaging position is applied. In thisfrictional engagement element, the friction plates are pressed by thepiston subjected to an urging force of the spring member, and hence thefriction plates are engaged with each other so as to have apredetermined engaging force. Note that, when a great engaging force isrequired, the piston is assisted by urging the piston with a hydraulicpressure.

In the frictional engagement element of Japanese Unexamined PatentPublication No. 2001-65607, the engaged state is produced solely by theurging force of the spring member. Thus, when the frictional engagementelement is switched from the released state to the engaged state, thereis a risk of delay in the switching of the state. In order to addressthe switching delay, it is conceived to move the piston at high speed byincreasing the urging force of the spring member. In this case, however,there is a risk of an engagement shock along with sudden engagement ofthe friction plates.

SUMMARY OF INVENTION

It is an object of the present invention to provide a frictionalengagement element in which a piston can promptly be moved from areleasing position toward an engaging position while preventing anengagement shock, and to provide an automatic transmission using thefrictional engagement element.

A frictional engagement element according to one aspect of the presentinvention for achieving this object includes: a plurality of frictionplates arranged with clearances therebetween; a piston movable between areleasing position where the friction plates are brought into a releasedstate, and an engaging position where the friction plates are broughtinto an engaged state by pressing the friction plates; and a firsturging mechanism and a second urging mechanism configured to urge thepiston in an engaging direction from the releasing position toward theengaging position.

Urging forces of both of the first urging mechanism and the secondurging mechanism act on the piston from the releasing position to afirst position in the engaging direction. The urging force of the firsturging mechanism acts on the piston from the first position to a secondposition closer to the engaging position than the first position. Thefirst position is a predetermined position where the piston does notabut against the friction plates, and the second position is a positionwhere the piston abuts against the friction plates and the clearancesare reduced.

An automatic transmission according to another aspect of the presentinvention includes: a gear shifting mechanism; and the frictionalengagement element described above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a skeleton diagram of an automatic transmission according toan embodiment of the present invention;

FIG. 2 is an engagement table of frictional engagement elements providedto the automatic transmission;

FIG. 3 is a schematic sectional view illustrating the configuration ofthe frictional engagement element;

FIG. 4 is a view illustrating an example of arrangement of first andsecond compression coil springs;

FIG. 5 is a schematic sectional view for describing an operation of thefrictional engagement element;

FIG. 6 is a schematic sectional view for describing the operation of thefrictional engagement element;

FIG. 7 is a schematic sectional view for describing the operation of thefrictional engagement element;

FIG. 8 is a schematic sectional view for describing the operation of thefrictional engagement element;

FIG. 9 is a schematic sectional view for describing the operation of thefrictional engagement element;

FIG. 10 is a block diagram illustrating hydraulic pressure routes of theautomatic transmission;

FIG. 11 is a diagram in the form of a table showing how hydraulicpressures are supplied to the frictional engagement elements when avehicle is started from a stopped state; and

FIG. 12 is a view illustrating a modified example of the arrangement ofthe first and second compression coil springs.

DESCRIPTION OF EMBODIMENTS

[Overall Configuration of Automatic Transmission]

FIG. 1 is a skeleton diagram illustrating the configuration of anautomatic transmission 1 for an automobile (vehicle) according to anembodiment of the present invention. The automatic transmission 1includes a transmission case 2, and an input shaft 3 extending from theengine side, an output gear 4, four planetary gear sets (first, second,third, and fourth planetary gear sets 11, 12, 13, and 14), two brakes(first and second brakes 21 and 22), and three clutches (first, second,and third clutches 31, 32, and 33), which are arranged in thetransmission case 2. The four planetary gear sets, the two brakes, andthe three clutches serve as a gear shifting mechanism.

The input shaft 3 is a shaft to which power generated by the engine isinput. The output gear 4 is a gear configured to output a drive forceadjusted to a predetermined gear ratio by the gear shifting mechanism.This embodiment exemplifies a so-called torque converter-less automatictransmission in which the power of the engine is input to the inputsection without intervention of a torque converter (hydraulic powertransmission device).

The transmission case 2 includes an outer peripheral wall 2 a, a firstintermediate wall 2 b provided at the end of the outer peripheral wall 2a on the engine side, a second intermediate wall 2 c provided on a sideopposite to the engine side with respect to the first intermediate wall2 b, a third intermediate wall 2d provided at the middle of the outerperipheral wall 2 a in an axial direction, a side wall 2 e provided atthe end of the outer peripheral wall 2 a on the side opposite to theengine side, a boss portion 2 f provided so as to extend from the centerof the side wall 2 e toward the engine side, and a cylindrical portion 2g provided so as to extend from the inner peripheral end of the secondintermediate wall 2 c toward the side opposite to the engine side.

The four planetary gear sets 11 to 14 are arranged in the order of thefirst planetary gear set 11, the inner peripheral second planetary gearset 12 and the outer peripheral third planetary gear set 13 that arearranged while being stacked on each other in a radial direction, andthe fourth planetary gear set 14 from the engine side. The firstplanetary gear set 11 includes a carrier 11 c, pinions (not shown)supported by the carrier 11 c, a sun gear 11 s, and a ring gear 11 r.The first planetary gear set 11 is a single-pinion planetary gear set inwhich the pinions directly mesh with the sun gear 11 s and the ring gear11 r. The second, third, and fourth planetary gear sets 12, 13, and 14are also single-pinion planetary gear sets, and include carriers 12 c,13 c, and 14 c, pinions (not shown), sun gears 12 s, 13 s, and 14 s, andring gears 12 r, 13 r, and 14 r, respectively.

The ring gear 12 r of the second planetary gear set 12 and the sun gear13 s of the third planetary gear set 13 that are arranged while beingstacked at two stages in the radial direction are integrated with eachother by welding, shrink fitting, or the like. That is, the ring gear 12r and the sun gear 13 s are constantly coupled to each other to form anintegral rotary element 15. The sun gear 11 s of the first planetarygear set 11 and the sun gear 12 s of the second planetary gear set 12,the ring gear 11 r of the first planetary gear set 11 and the carrier 14c of the fourth planetary gear set 14, and the carrier 11 c of the firstplanetary gear set 11 and the carrier 13 c of the third planetary gearset 13 are constantly coupled to each other, respectively. The inputshaft 3 is constantly coupled to the carrier 12 c of the secondplanetary gear set 12. The output gear 4 is constantly coupled to eachof the carrier 11 c of the first planetary gear set 11 and the carrier13 c of the third planetary gear set 13. The output gear 4 is supportedby the cylindrical portion 2g of the transmission case 2 throughintermediation of a bearing 41 in a freely rotatable manner.

A first rotary member 34 is coupled to the sun gear 14 s of the fourthplanetary gear set 14. The first rotary member 34 extends toward theside opposite to the engine side. Similarly, a second rotary member 35is coupled to the ring gear 13 r of the third planetary gear set 13, anda third rotary member 36 is coupled to the integral rotary element 15.The rotary members 35 and 36 also extend toward the side opposite to theengine side. A fourth rotary member 37 is coupled to the carrier 12 c ofthe second planetary gear set 12 through intermediation of the inputshaft 3.

The first brake 21 is arranged on the first intermediate wall 2 b of thetransmission case 2. The first brake 21 includes a cylinder 211, apiston 212 fitted to the cylinder 211, and a working hydraulic pressurechamber 213 defined by the cylinder 211 and the piston 212. By supplyinga predetermined engaging hydraulic pressure to the working hydraulicpressure chamber 213, friction plates of the first brake 21 are engagedto fix the sun gear 11 s of the first planetary gear set 11 and the sungear 12 s of the second planetary gear set 12 to the transmission case2.

The second brake 22 is arranged on the third intermediate wall 2d. Thesecond brake 22 includes a cylinder 23, a piston 24 fitted to thecylinder 23, and an engaging hydraulic pressure chamber 25 defined bythe cylinder 23 and the piston 24. By supplying a predetermined engaginghydraulic pressure to the engaging hydraulic pressure chamber 25,friction plates of the second brake 22 are engaged to fix the ring gear14 r of the fourth planetary gear set 14 to the transmission case 2.This embodiment describes an example in which a frictional engagementelement according to the present invention is applied to the secondbrake 22. The second brake 22 is described later in detail withreference to FIG. 3 and other subsequent figures.

The first to third clutches 31 to 33 are arranged in the transmissioncase 2 at the end located on the side opposite to the engine side. Thefirst to third clutches 31 to 33 are arranged at the same positions inthe axial direction while being stacked on each other in the radialdirection so that the second clutch 32 is located on an inner peripheralside of the first clutch 31 and the third clutch 33 is located on aninner peripheral side of the second clutch 32.

The first clutch 31 connects or disconnects the sun gear 14 s of thefourth planetary gear set 14 to or from the ring gear 13 r of the thirdplanetary gear set 13. In other words, the first clutch 31 switches aconnection state between the first rotary member 34 coupled to the sungear 14 s and the second rotary member 35 coupled to the ring gear 13 r.

The second clutch 32 connects or disconnects the sun gear 14 s of thefourth planetary gear set 14 to or from the integral rotary element 15(that is, the ring gear 12 r of the second planetary gear set 12 and thesun gear 13 s of the third planetary gear set 13). In other words, thesecond clutch 32 switches a connection state between the first rotarymember 34 coupled to the sun gear 14 s and the third rotary member 36coupled to the integral rotary element 15.

The third clutch 33 connects or disconnects the sun gear 14 s of thefourth planetary gear set 14 to or from the input shaft 3 and thecarrier 12 c of the second planetary gear set 12. In other words, thethird clutch 33 switches a connection state between the first rotarymember 34 coupled to the sun gear 14 s and the fourth rotary member 37coupled to the carrier 12 c through intermediation of the input shaft 3.

The connection state between the first rotary member 34 and the secondrotary member 35 is switched by the first clutch 31, the connectionstate between the first rotary member 34 and the third rotary member 36is switched by the second clutch 32, and the connection state betweenthe first rotary member 34 and the fourth rotary member 37 is switchedby the third clutch 33. That is, the first rotary member 34 is a commonrotary member on one side out of the two rotary members for which eachof the clutches 31 to 33 switches the connection state. Therefore, acommon rotary member 30 having a wall portion orthogonal to the axis isarranged in proximity to the side wall 2 e of the transmission case 2 onthe side opposite to the engine side with respect to the first to thirdclutches 31 to 33. The first rotary member 34 is coupled to the commonrotary member 30.

The common rotary member 30 is shared among the first to third clutches31 to 33, and cylinders, pistons, working hydraulic pressure chambers,working hydraulic pressure paths, centrifugal balance hydraulic pressurechambers, centrifugal balance chamber forming members, and the like thatare provided to the respective clutches 31 to 33 are supported by thecommon rotary member 30. FIG. 1 simply illustrates pistons 31 p, 32 p,and 33 p of the first, second, and third clutches 31, 32, and 33. Notethat a common member 38 configured to retain friction plates of thesecond clutch 32 and the third clutch 33 is assembled to those clutches.

As described above, the automatic transmission 1 of this embodimentincludes the gear shifting mechanism configured to change a gear ratiobetween the input shaft 3 and the output gear 4, including the first tofourth planetary gear sets 11 to 14, and the first and second brakes 21and 22 and the first to third clutches 31 to 33 that serve as fivefrictional engagement elements. FIG. 2 is an engagement table of thefive frictional engagement elements provided to the automatictransmission 1. As shown in the engagement table of FIG. 2, threefrictional engagement elements out of the five frictional engagementelements are selectively engaged (symbols “O”) to achieve first toeighth forward gears and a reverse gear. In FIG. 2, CL1, CL2, and CL3denote the first, second, and third clutches 31 to 33, and BR1 and BR2denote the first and second brakes 21 and 22, respectively. In thisembodiment, the first clutch 31 (CL1), the first brake 21 (BR1), and thesecond brake 22 (BR2) are frictional engagement elements for achieving astarting gear ratio at the time of forward driving of the vehicle.

[Details of Frictional Engagement Element]

FIG. 3 is a schematic sectional view illustrating the configuration ofthe frictional engagement element of the gear shifting mechanismaccording to the embodiment of the present invention. The gear shiftingmechanism changes the gear ratio between the input shaft 3 and theoutput gear 4. In this case, description is given of an example in whichthe frictional engagement element is applied to the second brake 22. InFIG. 3 (and FIGS. 4 to 9 subsequent thereto), the axial direction of theinput shaft 3 is represented by an X direction, and the radial directionof the automatic transmission 1 is represented by a Y direction.Further, regarding the X direction, the left side of the figure isrepresented by “−X”, and the right side of the figure is represented by“+X” for convenience.

The second brake 22 includes a housing 20, the piston 24 and theengaging hydraulic pressure chamber 25 described above, a releasinghydraulic pressure chamber 26, a ring member 27, a friction plate unit 5(plurality of friction plates), first compression coil springs 61 (firsturging mechanism), and second compression coil springs 62 (second urgingmechanism). A hydraulic mechanism 70 is annexed to the second brake 22described above. The hydraulic mechanism 70 includes an oil pump 71, ahydraulic pressure circuit 72, and a hydraulic pressure control section75.

The housing 20 is a part of the transmission case 2 illustrated in FIG.1, and includes an outer tubular portion 201, a flange portion 202, andan inner tubular portion 203. The outer tubular portion 201 is a tubularportion extending in the axial direction of the input shaft 3 (FIG. 1),and is a portion corresponding to the outer peripheral wall 2 a of thetransmission case 2. The flange portion 202 and the inner tubularportion 203 are portions corresponding to the intermediate wall 2d ofthe transmission case 2. The flange portion 202 is a portion extendingradially inward from the outer tubular portion 201. The inner tubularportion 203 is arranged on a radially inner side of the outer tubularportion 201 at a predetermined distance from the outer tubular portion201. A space formed by the outer tubular portion 201, the flange portion202, and the inner tubular portion 203 constitutes a space of thecylinder 23 of the second brake 22 described above.

A groove portion 204 is formed in a region adjacent to the flangeportion 202 on the inner surface of the outer tubular portion 201. Thus,a receiving surface 205 opposed to an inner surface 20A of the flangeportion 202 at a predetermined distance in the axial direction is formedon the inner surface side of the outer tubular portion 201. The secondcompression coil spring 62 is accommodated in the groove portion 204.The inner tubular portion 203 is provided with a first supply port 206for supplying a hydraulic pressure to the engaging hydraulic pressurechamber 25, and a second supply port 207 for supplying a hydraulicpressure to the releasing hydraulic pressure chamber 26.

The piston 24 is a member movable in the axial direction between theouter tubular portion 201 and the inner tubular portion 203, andincludes a pressing piece 241 configured to slide against the innerperipheral surface of the outer tubular portion 201, and a pressurereceiving piece 242 opposed to the flange portion 202. The pressingpiece 241 has a tip end surface 24A located on a tip end side (+X side)in a moving direction and configured to apply a pressing force to thefriction plate unit 5. Further, the pressure receiving piece 242 has arear end surface 24B located on a rear end side (−X side) and configuredto receive a pressing force of the hydraulic pressure and urging forcesof the first and second compression coil springs 61 and 62. The piston24 moves between a releasing position (position illustrated in FIG. 5)where the friction plate unit 5 is brought into a released state, and anengaging position (position illustrated in FIG. 9) where the frictionplate unit 5 is brought into an engaged state by applying a pressingforce to the friction plate unit 5.

The engaging hydraulic pressure chamber 25 is a space to be suppliedwith a hydraulic pressure for moving the piston 24 in a direction towardthe engaging position. The engaging hydraulic pressure chamber 25 is aspace defined by the outer tubular portion 201, the flange portion 202,and the inner tubular portion 203 of the housing 20 and the pressingpiece 241 of the piston 24. In this embodiment, the first and secondcompression coil springs 61 and 62 are arranged in the engaginghydraulic pressure chamber 25.

The releasing hydraulic pressure chamber 26 is a space to be suppliedwith a hydraulic pressure for moving the piston 24 in a direction towardthe releasing position. The releasing hydraulic pressure chamber 26 is aspace defined by the inner tubular portion 203 of the housing 20, thepressing piece 241 and the pressure receiving piece 242 of the piston24, and the ring member 27 assembled to the inner tubular portion 203.In this embodiment, a return spring or the like is not arranged in thereleasing hydraulic pressure chamber 26.

The friction plate unit 5 includes a plurality of friction platesarranged with clearances therebetween. Specifically, the friction plateunit 5 includes a plurality of drive plates 51 and a plurality of drivenplates 52 alternately arrayed with predetermined clearances Ctherebetween. Facings are bonded to both surfaces of the drive plate 51.The drive plates 51 are spline-coupled to a first spline piece 53, andthe driven plates 52 are spline-coupled to a second spline piece 54. Thefirst spline piece 53 is a member corresponding to an outer peripheralportion of the ring gear 14 r of the fourth planetary gear set 14illustrated in FIG. 1. Further, the second spline piece 54 is a membercorresponding to an inner peripheral portion of the outer peripheralwall 2 a of the transmission case 2.

The tip end surface 24A of the piston 24 abuts against the driven plate52 located farthest to the −X side to apply a pressing force to thefriction plate unit 5. A retaining plate 55 is arranged adjacent to thedrive plate 51 located farthest to the +X side. The retaining plate 55regulates movement of the drive plates 51 and the driven plates 52 inthe +X direction.

The first compression coil spring 61 and the second compression coilspring 62 are springs configured to urge the piston 24 in an engagingdirection (+X direction) from the releasing position toward the engagingposition. The first and second compression coil springs 61 and 62 arearranged on the surface of the piston 24 which is opposite to thesurface opposed to the friction plate unit 5. The ends of the first andsecond compression coil springs 61 and 62 on the −X side are supportedby the inner surface 20A of the immovable flange portion 202, and theends on the +X side abut against the rear end surface 24B of the piston24 configured to move in the X direction. Note that the secondcompression coil spring 62 is arranged so as to be accommodated in thegroove portion 204. Thus, the receiving surface 205 of the grooveportion 204 serves as a regulating surface against which the end of thesecond compression coil spring 62 on the +X side abuts, therebyregulating stretching of the second compression coil spring 62 in the +Xdirection.

On the other hand, such a regulating surface is not provided for thefirst compression coil spring 61, and the first compression coil spring61 is stretchable to have a free length. The free length of the firstcompression coil spring 61 is larger than the distance between the innersurface 20A and the receiving surface 205. That is, the working lengthof the first compression coil spring 61 that applies an urging force tothe piston 24 in the +X direction is larger than the working length ofthe second compression coil spring 62. Thus, the urging forces of bothof the first and second compression coil springs 61 and 62 act on thepiston 24 up to a position of the receiving surface 205 (first position;position of FIG. 6 described later) from a state in which the piston 24is located at the releasing position, and only the urging force of thefirst compression coil spring 61 acts on the piston 24 up to a positionwhere the first compression coil spring 61 stretches from the receivingsurface 205 to have a free length (second position; position of FIG. 8described later).

Further, the second compression coil spring 62 is a larger spring thanthe first compression coil spring 61. That is, the urging force of thesecond compression coil spring 62 is set greater than the urging forceof the first compression coil spring 61. Thus, an assist of therelatively greater urging force of the second compression coil spring 62is superimposed on the urging force of the first compression coil spring61 up to the position of the receiving surface 205 from the releasingposition, and hence the moving speed of the piston 24 is increased.

FIG. 4 is a view illustrating an example of arrangement of the first andsecond compression coil springs 61 and 62. The pluralities of first andsecond compression coil springs 61 and 62 are each arrayed in an annularform on the disc-like (annular) rear end surface 24B of the piston 24.The first compression coil springs 61 are arranged on a radially innerside of the piston 24, and the second compression coil springs 62 arearranged on a radially outer side of the piston 24. End plates 63 areattached to the ends of the first compression coil springs 61 on the +Xside and the −X side, respectively. Similarly, end plates 64 areattached to the ends of the second compression coil springs 62 on the +Xside and the −X side, respectively. With the end plates 63 and 64, theurging forces of the first and second compression coil springs 61 and 62can be applied to the piston 24 stably and uniformly in acircumferential direction.

The arrangement of the first and second compression coil springs 61 and62 as illustrated in FIG. 4 is an arrangement capable of easilyincreasing an urging force of a group of the plurality of secondcompression coil springs 62. Specifically, the second compression coilsprings 62 are arranged on the radially outer side of the piston 24, anda wider arrangement space can be secured for the second compression coilsprings 62 than for the first compression coil springs 61 because thecircumferential length is larger on the radially outer side than on theradially inner side. Thus, larger compression coil springs than thefirst compression coil springs 61 are employed as the second compressioncoil springs 62 or a larger number of second compression coil springs 62than the first compression coil springs 61 are arranged even if thesizes are the same, thereby being capable of easily producing asituation in which the urging force of the group of second compressioncoil springs 62 is greater than the urging force of the group of firstcompression coil springs 61.

The oil pump 71 of the hydraulic mechanism 70 is a pump to be driven bythe engine so as to cause oil to flow through desired portions andgenerate predetermined hydraulic pressures. The hydraulic pressurecircuit 72 is a hydraulic pressure circuit for achieving the respectivegear positions shown in FIG. 2 by selectively supplying the hydraulicpressures to the first and second brakes 21 and 22 and the first tothird clutches 31 to 33 that serve as the frictional engagementelements. FIG. 3 only illustrates a first solenoid valve 73 and a secondsolenoid valve 74 for supplying and discharging the hydraulic pressuresto and from the second brake 22.

The first solenoid valve 73 is a valve configured to control the supplyof the hydraulic pressure generated by the oil pump 71 to the engaginghydraulic pressure chamber 25 through the first supply port 206 and thedischarge of the hydraulic pressure from the engaging hydraulic pressurechamber 25. The first solenoid valve 73 includes an input port 731, anoutput port 732, and a drain port 733. Through an operation of thesolenoid, the input port 731 and the output port 732 are caused tocommunicate with each other when the hydraulic pressure is supplied tothe engaging hydraulic pressure chamber 25, and the output port 732 andthe drain port 733 are caused to communicate with each other when thehydraulic pressure is discharged. When a predetermined engaginghydraulic pressure is supplied to the engaging hydraulic pressurechamber 25, the piston 24 moves to the engaging position.

The second solenoid valve 74 is a valve configured to control the supplyof the hydraulic pressure generated by the oil pump 71 to the releasinghydraulic pressure chamber 26 through the second supply port 207 and thedischarge of the hydraulic pressure from the releasing hydraulicpressure chamber 26. The second solenoid valve 74 includes an input port741, an output port 742, and a drain port 743. Through an operation ofthe solenoid, the input port 741 and the output port 742 are caused tocommunicate with each other when the hydraulic pressure is supplied tothe releasing hydraulic pressure chamber 26, and the output port 742 andthe drain port 743 are caused to communicate with each other when thehydraulic pressure is discharged. When a predetermined releasinghydraulic pressure is supplied to the releasing hydraulic pressurechamber 26, the piston 24 moves to the releasing position irrespectiveof the urging forces of the first and second compression coil springs 61and 62.

The hydraulic pressure control section 75 controls the hydraulicpressures to be supplied to the engaging hydraulic pressure chamber 25and the releasing hydraulic pressure chamber 26 by controlling theoperations of the solenoids of the first and second solenoid valves 73and 74. In addition, the hydraulic pressure control section 75 controlsthe respective solenoid valves and the like of the other frictionalengagement elements, and also controls the hydraulic pressures to besupplied to the first brake 21 and the first to third clutches 31 to 33.

[Operation of Frictional Engagement Element]

FIGS. 5 to 9 are schematic sectional views for describing an operationof the frictional engagement element. FIG. 5 illustrates a state inwhich the piston 24 is located at the releasing position. Specifically,the tip end surface 24A of the piston 24 is spaced away from thefriction plate unit 5 by a predetermined distance, and the drive plates51 and the driven plates 52 are in the released state. FIG. 5illustrates the position of the tip end surface 24A in the X directionas a releasing position P0. At this time, the hydraulic pressure controlsection 75 prevents the engaging hydraulic pressure from being suppliedto the engaging hydraulic pressure chamber 25 (represented by “OFF” inFIG. 5). That is, the hydraulic pressure control section 75 controls thefirst solenoid valve 73 into a state in which the output port 732 andthe drain port 733 are caused to communicate with each other.

On the other hand, the hydraulic pressure control section 75 causes apredetermined releasing hydraulic pressure to be supplied to thereleasing hydraulic pressure chamber 26 through the second supply port207 (represented by “ON” in FIG. 5). That is, the hydraulic pressurecontrol section 75 controls the second solenoid valve 74 into a state inwhich the input port 741 and the output port 742 are caused tocommunicate with each other. The releasing hydraulic pressure is ahydraulic pressure at least greater than the urging force of the firstcompression coil spring 61, and is a hydraulic pressure greater than thesuperimposed urging force of the first and second compression coilsprings 61 and 62 in this embodiment. The pressure receiving piece 242of the piston 24 is supplied with the releasing hydraulic pressure, andhence the piston 24 moves in the −X direction against the superimposedurging force. The first and second compression coil springs 61 and 62are brought into the most compressed state.

FIG. 6 illustrates a state in which the piston 24 is located at thefirst position. The first position is set to a predetermined positionwhere the tip end surface 24A of the piston 24 does not abut against thefriction plate unit 5. Specifically, the first position is determined bythe width of the groove portion 204 in the X direction. FIG. 6illustrates the position of the tip end surface 24A in the X directionat this time as a first position P1. When a command to move the piston24 from the releasing position P0 in the engaging direction is issued,the hydraulic pressure control section 75 turns OFF the supply of thereleasing hydraulic pressure to the releasing hydraulic pressure chamber26 (causes the hydraulic pressure to be discharged). The engaginghydraulic pressure chamber 25 is maintained in the OFF state.

By maintaining the releasing hydraulic pressure in the OFF state, thepiston 24 is brought into a state in which the piston 24 is freelymovable in the +X direction. When the piston 24 then moves from thereleasing position P0 to the first position P1, the piston 24 issubjected to the urging forces of both of the first and secondcompression coil springs 61 and 62 in the superimposed state. Thus, thepiston 24 is forcefully pressed in the +X direction to move in theengaging direction at high speed. That is, the piston 24 can start tomove in the engaging direction quickly.

FIG. 7 illustrates a state in which the piston 24 has further moved fromthe first position P1 in the engaging direction by a predetermineddistance. In this state, only the urging force of the first compressioncoil spring 61 out of the two compression coil springs acts on thepiston 24. FIG. 7 illustrates the position of the tip end surface 24A inthe X direction at this time as an immediate pre-contact position P11.

At a timing when the piston 24 has reached the first position P1, thehydraulic pressure control section 75 causes a pre-charge hydraulicpressure lower than the engaging hydraulic pressure (predeterminedhydraulic pressure) to be supplied to the engaging hydraulic pressurechamber 25. The releasing hydraulic pressure chamber 26 is maintained inthe OFF state. Through the sudden movement of the piston 24 from thereleasing position PO to the first position P1, the engaging hydraulicpressure chamber 25 has a negative pressure inside. The negativepressure hinders the movement of the piston 24 in the +X direction.Therefore, the pre-charge hydraulic pressure is supplied to the engaginghydraulic pressure chamber 25 to assist the movement of the piston 24 inthe +X direction. Note that the pre-charge hydraulic pressure is such ahydraulic pressure as to remove the negative pressure resistance, andthe movement of the piston 24 from the first position P1 to a nextsecond position P2 (FIG. 8) depends solely on the urging force of thefirst compression coil spring 61. Thus, the piston 24 moves relativelyslowly from the first position P1 to the second position P2 as comparedto the case in which the piston 24 moves from the releasing position P0to the first position P1.

FIG. 8 illustrates a state in which the piston 24 is located at thesecond position where the tip end surface 24A thereof abuts against thefriction plate unit 5 (driven plate 52) and the clearances C between thedrive plates 51 and the driven plates 52 are reduced. At the secondposition, the first compression coil spring 61 is brought into the moststretched state, but the piston 24 has not reached the engagingposition. FIG. 8 illustrates the position of the tip end surface 24A inthe X direction at this time as the second position P2. During a periodin which the piston 24 moves from the immediate pre-contact position P11to the second position P2, the hydraulic pressure control section 75stops the supply of the pre-charge hydraulic pressure to the engaginghydraulic pressure chamber 25. Thus, at the second position P2, thesupply of the hydraulic pressures to both of the engaging hydraulicpressure chamber 25 and the releasing hydraulic pressure chamber 26 isturned OFF.

The second position P2 corresponds to a starting preparation state inwhich the vehicle can be started at any time, and corresponds to aso-called zero-touch state. The zero-touch state refers to a state inwhich the clearances C between the drive plates 51 and the driven plates52 are reduced to the extent close to establishment of frictionalengagement therebetween, and a frictional engagement force is generatedtherebetween when the piston 24 is further moved in the +X direction. Inthis embodiment, the zero-touch state can stably be produced by theurging force of the first compression coil spring 61, and can bemaintained as well. In other words, the working length of the firstcompression coil spring 61 is selected so that the zero-touch state canbe produced.

FIG. 9 illustrates a state in which the piston 24 has reached theengaging position. Specifically, the tip end surface 24A of the piston24 presses the driven plate 52 in the +X direction, and the drive plates51 and the driven plates 52 are brought into the engaged state. FIG. 9illustrates the position of the tip end surface 24A in the X directionat this time as an engaging position P3. At this time, the engaginghydraulic pressure chamber 25 is brought into an ON state in which theengaging hydraulic pressure is supplied. The releasing hydraulicpressure chamber 26 is maintained in the OFF state.

When a starting instruction on forward or reverse driving is issuedwhile the piston 24 is located at the second position P2 in thezero-touch state, the hydraulic pressure control section 75 causes theengaging hydraulic pressure to be supplied to the engaging hydraulicpressure chamber 25. The pressure receiving piece 242 receives theengaging hydraulic pressure, and the piston 24 further moves from thesecond position P2 in the +X direction to reach the engaging positionP3. Thus, the drive plates 51 and the driven plates 52 are engaged witheach other, and an engaging force is generated therebetween.

When the drive plates 51 and the driven plates 52 are disengaged fromeach other, the hydraulic pressure control section 75 brings theengaging hydraulic pressure chamber 25 into an OFF state, and brings thereleasing hydraulic pressure chamber 26 into an ON state in which thereleasing hydraulic pressure is supplied thereto. Through this control,the piston 24 returns to the releasing position P0 illustrated in FIG.5.

[Control to be Executed when Vehicle is Started]

Next, description is given of hydraulic pressure control to be executedin the automatic transmission 1 when the vehicle is started. FIG. 10 isa block diagram illustrating hydraulic pressure routes of the automatictransmission 1, and FIG. 11 is a diagram in the form of a table showinghow the hydraulic pressures are supplied to the respective frictionalengagement elements when the vehicle is started from a stopped state.FIG. 3 illustrates the hydraulic pressure circuit 72 corresponding tothe second brake 22, but FIG. 10 illustrates the hydraulic pressurecircuit 72 corresponding to all the frictional engagement elements(first and second brakes 21 and 22 and first to third clutches 31 to33). The hydraulic pressure circuit 72 includes solenoid valves forsupplying and discharging the hydraulic pressures to and from therespective frictional engagement elements.

As shown in FIG. 2, frictional engagement elements to be engaged whenthe vehicle is started from the stopped state, that is, when a gearratio of the first forward gear (starting gear ratio) is achieved arethe first clutch 31 (CL1), the first brake 21 (BR1), and the secondbrake 22 (BR2) in this embodiment. Thus, FIG. 11 only shows those threefrictional engagement elements in the table. In the boxes on the firstclutch 31 and the first brake 21 of FIG. 11, “ON” indicates that thefirst clutch 31 and the first brake 21 are in the engaged state, and“OFF” indicates that the first clutch 31 and the first brake 21 are inthe disengaged state (released state). In the boxes on the second brake22, “ON” indicates a state in which the engaging hydraulic pressure orthe releasing hydraulic pressure is supplied, and “OFF” indicates astate in which the engaging hydraulic pressure or the releasinghydraulic pressure is not supplied.

Referring to FIG. 11, when the vehicle is stopped, the hydraulicpressure control section 75 brings the first clutch 31 and the firstbrake 21 into an OFF state in which the engaging hydraulic pressure isnot supplied thereto. Further, the hydraulic pressure control section 75brings the engaging hydraulic pressure chamber 25 into an OFF state, andbrings the releasing hydraulic pressure chamber 26 into an ON state inwhich a predetermined releasing hydraulic pressure is supplied thereto(state illustrated in FIG. 5).

Next, the vehicle is switched from the stopped state to a startingpreparation state. The starting preparation state of the vehicle refersto a state in which the vehicle is in the stopped state but thezero-touch state described above is produced. In the zero-touch state,the hydraulic pressure control section 75 brings the first clutch 31 andthe first brake 21 into an ON state in which the engaging hydraulicpressure is supplied thereto. For the second brake 22, on the otherhand, the hydraulic pressure control section 75 brings, as describedabove, the engaging hydraulic pressure chamber 25 into an OFF state inwhich the engaging hydraulic pressure is not supplied thereto, andbrings the releasing hydraulic pressure chamber 26 into an OFF state inwhich the releasing hydraulic pressure thereof is released (stateillustrated in FIG. 8).

Specifically, in the starting preparation state (stopped state), thehydraulic pressure control section 75 brings the engaging hydraulicpressure chamber 25 of the second brake 22 into a state in which ahydraulic pressure lower than the engaging hydraulic pressure issupplied (OFF state), and brings the first clutch 31 and the first brake21, which are the other frictional engagement elements for achieving thestarting gear ratio, into a state in which the engaging hydraulicpressure is supplied (ON state). Thus, there is produced a state inwhich the starting gear ratio is achieved by switching the second brake22 to the engaged state lastly. Moreover, in the zero-touch state, thepiston 24 presses the friction plate unit 5 with the urging force of thefirst compression coil spring 61, thereby producing a state in which theclearances C between the drive plates 51 and the driven plates 52 arereduced.

When a starting instruction is issued in the starting preparation state(starting state), the hydraulic pressure control section 75 also bringsthe engaging hydraulic pressure chamber 25 of the second brake 22 intoan ON state in which the engaging hydraulic pressure is supplied theretoas well as the first clutch 31 and the first brake 21. Thus, thestarting gear ratio is achieved, and the vehicle travels forward. Inthis manner, the switching from the starting preparation state to thestarting state is achieved only by engaging the second brake 22 that hasalready been in the zero-touch state, and hence the response to achievethe starting gear ratio from the stopped state is increased greatly.Note that, when the starting instruction is not issued in the startingpreparation state, the second brake 22 waits in the zero-touch state,and when a stopping instruction is issued, a hydraulic pressure state of“STOP” in FIG. 11 is produced.

In this embodiment, the reason why the second brake 22 is selected asthe frictional engagement element configured to produce the zero-touchstate with the urging force of the compression coil spring is asfollows. As shown in FIG. 2, the gear shifting mechanism of thisembodiment is a stepped gear shifting mechanism ranging from the firstgear at which the starting gear ratio is achieved to the eighth gear atwhich the top speed gear ratio is achieved in the forward driving. Thesecond brake 22 is a frictional engagement element which is continuouslybrought into the engaged state so as to achieve the gear ratios of thefirst gear to the fifth gear out of the eight gears. In contrast, in therange from the first gear to the fifth gear, the first clutch 31 is africtional engagement element which is not engaged at the second gearand the fourth gear, and the first brake 21 is a frictional engagementelement which is not engaged at the third gear to the fifth gear.

For this reason, it can be said that the second brake 22 is a frictionalengagement element having the highest versatility at low-speed tomiddle-speed gear ratios. That is, the second brake 22 is a frictionalengagement element which is constantly engaged when the low-speed tomiddle-speed gear ratios are achieved on the basis of a predeterminedgear shift map. For example, in a scene in which the vehicle is suddenlydecelerated and then accelerated again, the second brake 22 isconstantly engaged even when a gear ratio of any one of the first gearto the fifth gear is achieved on the basis of the gear shift map. Thus,by selecting the second brake 22 as the frictional engagement elementconfigured to produce the zero-touch state, gear shifting that isexcellent in the response and causes no engagement shock can beachieved.

[Actions and Effects]

With the frictional engagement element or the automatic transmissionaccording to this embodiment described above, the following actions andeffects are attained. The second brake 22 to which the frictionalengagement element according to this embodiment is applied includes thefirst and second compression coil springs 61 and 62 configured to urgethe piston 24 in the engaging direction from the releasing position P0(FIG. 5) toward the engaging position P3 (FIG. 9). Further, from thereleasing position P0 to the first position P1 (FIG. 6), the urgingforces of both of the first and second compression coil springs 61 and62 act on the piston 24, and hence the piston 24 can quickly be moved inthe engaging direction. That is, the piston 24 can start to move in theengaging direction quickly.

Moreover, the piston 24 is subjected to the urging force of the firstcompression coil spring 61 so as to move from the first position P1 tothe second position P2 (FIG. 8). When the piston 24 has reached thesecond position P2, the clearances C between the drive plates 51 and thedriven plates 52 (between the friction plates) are reduced.Specifically, the plates 51 and 52 are not brought into the engagedstate, but the clearances between the plates 51 and 52 are substantiallyeliminated, and the plates 51 and 52 are brought into a state close toengagement. Thus, by supplying the engaging hydraulic pressure to theengaging hydraulic pressure chamber 25 afterwards, both the plates 51and 52 can promptly be brought into the engaged state. In this manner,the state close to engagement is produced by the urging force of thefirst compression coil spring 61, and the engaged state is producedafterwards. Accordingly, the occurrence of the engagement shock can beprevented. Further, the zero-touch state can stably be produced by thefirst compression coil spring 61.

Further, the movement of the piston 24 to the first position P1 and thesecond position P2 that are different positions in the +X directiondepends on a difference between the working lengths of the first andsecond compression coil springs 61 and 62. Specifically, the workinglength of the first compression coil spring 61 is set larger than theworking length of the second compression coil spring 62, and the urgingforce is applied to the piston 24 up to the first position P1 or thesecond position P2. Thus, a frictional engagement element that attainsthe actions and effects of the present invention can easily beconstructed by setting the working lengths of the first and secondcompression coil springs 61 and 62.

Moreover, the urging force of the second compression coil spring 62 isgreater than the urging force of the first compression coil spring 61.Thus, the moving speed of the piston 24 from the releasing position P0to the first position P1, on which the urging forces of both of thefirst and second compression coil springs 61 and 62 act, can be maderelatively higher than the moving speed of the piston from the firstposition P1 to the second position P2. That is, the piston 24 can startto move quickly, and hence the piston 24 can move toward the engagingposition P3 more promptly.

The first compression coil springs 61 are arranged on the radially innerside of the piston 24, and the second compression coil springs 62 arearranged on the radially outer side of the piston 24. A widerarrangement space can be secured for the second compression coil springs62 than for the first compression coil springs 61 because thecircumferential length is larger on the radially outer side than on theradially inner side. Thus, it is possible to easily produce a situationin which the urging force of the group of the plurality of secondcompression coil springs 62 is greater than the urging force of thegroup of the plurality of first compression coil springs 61.

In the hydraulic pressure control of the frictional engagement element,when the piston 24 is moved toward the releasing position P0, thehydraulic pressure control section 75 causes a hydraulic pressure atleast greater than the urging force of the first compression coil spring61 to be supplied to the releasing hydraulic pressure chamber 26. In theembodiment, the hydraulic pressure control section 75 causes a releasinghydraulic pressure greater than the urging forces of both of the firstand second compression coil springs 61 and 62 to be supplied to thereleasing hydraulic pressure chamber 26. Thus, the piston 24 canpromptly be returned toward the releasing position P0 against thesuperimposed urging force of the first and second compression coilsprings 61 and 62.

Further, when the piston 24 is moved in the engaging direction, thehydraulic pressure control section 75 causes the releasing hydraulicpressure in the releasing hydraulic pressure chamber 26 to bedischarged. Thus, the force in the direction toward the releasingposition P0 does not act on the piston 24, and hence the piston 24 canpromptly be moved in the engaging direction by the superimposed urgingforce of the first and second compression coil springs 61 and 62 whenthe piston 24 is moved to the first position P1, and by the urging forceof the first compression coil spring 61 when the piston 24 is moved tothe second position P2. That is, it is possible to promptly produce astate in which the clearances C between the plates 51 and 52 arereduced.

Moreover, when the piston 24 is moved in the engaging direction, thehydraulic pressure control section 75 causes the pre-charge hydraulicpressure to be supplied to the engaging hydraulic pressure chamber 25(FIG. 7). Thus, the piston 24 can be moved in the engaging direction athigher speed by the pre-charge hydraulic pressure in addition to theurging forces of the first and second compression coil springs 61 and62. Further, the supply of the pre-charge hydraulic pressure can addressthe problem in that the piston 24 is quickly moved in the engagingdirection by the urging forces of the first and second compression coilsprings 61 and 62 so that the engaging hydraulic pressure chamber 25 hasa negative pressure and the moving speed of the piston 24 is decreased.

As described above, according to this embodiment, it is possible toprovide the frictional engagement element in which the piston 24 canpromptly be moved from the releasing position P0 toward the engagingposition while preventing the engagement shock, and to provide theautomatic transmission 1 using the frictional engagement element.

[Description of Modified Embodiments]

One embodiment of the present invention is described above, but thepresent invention is not limited thereto, and the following modifiedembodiments may be employed.

(1) In the embodiment, the first compression coil spring 61 and thesecond compression coil spring 62 are exemplified as the first urgingmechanism and the second urging mechanism, respectively. Other urgingmembers may be applied as the first and second urging mechanisms as longas the urging force in the engaging direction can be applied to thepiston 24 and the urging members can be incorporated in the transmissioncase 2. For example, a tension coil spring, a belleville spring, a flatspring, and an elastic rubber or resin may be used as the first andsecond urging mechanisms.

(2) In the embodiment, as illustrated in FIG. 4, there is described anexample in which the second compression coil spring 62 is a largerspring than the first compression coil spring 61. FIG. 12 is a viewillustrating a modified example of the arrangement of the compressioncoil springs. FIG. 12 illustrates an example using a second compressioncoil spring 62A having the same size as the first compression coilspring 61. Note that the density of array of the second compression coilsprings 62A on the radially outer side is set higher than the density ofarray of the first compression coil springs 61 on the radially innerside. In this manner, the urging environment may be produced so that theurging force of the group of second compression coil springs 62A isgreater than the urging force of the group of first compression coilsprings 61.

(3) In the embodiment, there is described an example in which thefrictional engagement element configured to produce the zero-touch statewith the urging force of the first compression coil spring 61 is thesecond brake 22. In place of or in addition to the second brake 22, anyone or a plurality of the other frictional engagement elements, that is,the first brake 21 and the first to third clutches 31 to 33 may beselected as the frictional engagement element configured to produce thezero-touch state.

(4) FIGS. 10 and 11 exemplify a case in which the starting gear ratio offorward driving is achieved. When the starting gear ratio of reversedriving is achieved, the combination of the first clutch 31, the firstbrake 21, and the second brake 22 is replaced with a combination of thethird clutch 33, the first brake 21, and the second brake 22 as shown inthe engagement table of FIG. 2.

(5) In the embodiment, there is described an example in which thepre-charge hydraulic pressure is supplied to the engaging hydraulicpressure chamber 25 at a timing when the piston 24 has reached the firstposition P1. As a measure against the resistance to the piston 24 alongwith the generation of the negative pressure in the engaging hydraulicpressure chamber 25, elastic forces of various elastic members may beused instead of the supply of the pre-charge hydraulic pressure.

(6) In the embodiment, there is described an example in which the firstand second compression coil springs 61 and 62 are arranged in theengaging hydraulic pressure chamber 25 as the urging mechanisms. Inplace of this example, an urging mechanism such as a tension coil springmay be arranged in the releasing hydraulic pressure chamber 26.

Note that the specific embodiments described above disclose a frictionalengagement element having the following configurations and an automatictransmission using the frictional engagement element.

A frictional engagement element according to one aspect of the presentinvention includes: a plurality of friction plates arranged withclearances therebetween; a piston movable between a releasing positionwhere the friction plates are brought into a released state, and anengaging position where the friction plates are brought into an engagedstate by pressing the friction plates; and a first urging mechanism anda second urging mechanism configured to urge the piston in an engagingdirection from the releasing position toward the engaging position.Urging forces of both of the first urging mechanism and the secondurging mechanism act on the piston from the releasing position to afirst position in the engaging direction. The urging force of the firsturging mechanism acts on the piston from the first position to a secondposition closer to the engaging position than the first position. Thefirst position is a predetermined position where the piston does notabut against the friction plates, and the second position is a positionwhere the piston abuts against the friction plates and the clearancesare reduced.

According to the frictional engagement element, from the releasingposition to the first position, the urging forces of both of the firstand second urging mechanisms act on the piston, and hence the piston canquickly be moved in the engaging direction. Moreover, the piston issubjected to the urging force of the first urging mechanism so as tomove from the first position to the second position. When the piston hasreached the second position, the clearances of the friction plates arereduced. Thus, by causing the hydraulic pressure or the like to act onthe piston afterwards, the friction plates can promptly be brought intothe engaged state. Further, the engaged state is not produced at thesecond position, and hence the occurrence of the engagement shock can beprevented.

In the frictional engagement element, it is desired that: the firsturging mechanism be formed of a first compression coil spring, and thesecond urging mechanism be formed of a second compression coil spring;and a working length of the first compression coil spring be larger thana working length of the second compression coil spring.

According to the frictional engagement element, the application of theurging force to the piston up to the first position or the secondposition can be set by the working lengths of the first and secondcompression coil springs. Thus, the frictional engagement element caneasily be constructed.

In the frictional engagement element, it is desired that the urgingforce of the second urging mechanism be greater than the urging force ofthe first urging mechanism.

According to the frictional engagement element, the moving speed of thepiston from the releasing position to the first position, on which theurging forces of both of the first and second urging mechanisms act, canbe made relatively higher than the moving speed of the piston from thefirst position to the second position. That is, the piston can start tomove quickly, and hence the piston can move toward the engaging positionmore promptly.

In this case, it is desired that: the piston have a disc-like shape; andthe first urging mechanism be arranged on a radially inner side of thepiston, and the second urging mechanism be arranged on a radially outerside of the piston.

According to the frictional engagement element, the second urgingmechanism is arranged on the radially outer side of the piston. Thecircumferential length is larger on the radially outer side than on theradially inner side, and hence a larger number of urging members can bearranged. Thus, it is possible to easily produce a situation in whichthe urging force of the second urging mechanism is greater than theurging force of the first urging mechanism.

It is desired that: the frictional engagement element further include: areleasing hydraulic pressure chamber configured to move the piston in adirection toward the releasing position; and a hydraulic pressurecontrol section configured to control a hydraulic pressure to besupplied to the releasing hydraulic pressure chamber; and when thepiston is moved toward the releasing position, the hydraulic pressurecontrol section cause a hydraulic pressure at least greater than anurging force of the first urging mechanism to be supplied to thereleasing hydraulic pressure chamber.

According to the frictional engagement element, the piston can promptlybe returned toward the releasing position against the urging force ofthe first urging mechanism by the hydraulic pressure supplied to thereleasing hydraulic pressure chamber.

In the frictional engagement element, it is desired that, when thepiston is moved in the engaging direction, the hydraulic pressurecontrol section cause the hydraulic pressure in the releasing hydraulicpressure chamber to be discharged.

According to the frictional engagement element, the force in thedirection toward the releasing position does not act on the piston, andhence the piston can promptly be moved in the engaging direction by theurging forces of both of the first and second urging mechanisms when thepiston is moved to the first position, and by the urging force of thefirst urging mechanism when the piston is moved to the second position.That is, it is possible to promptly produce a state in which theclearances are reduced.

It is desired that: the frictional engagement element further include anengaging hydraulic pressure chamber configured to move the piston in adirection toward the engaging position; and when the piston is moved inthe engaging direction, the hydraulic pressure control section cause apredetermined hydraulic pressure to be supplied to the engaginghydraulic pressure chamber.

According to the frictional engagement element, the piston can be movedin the engaging direction at higher speed by the hydraulic pressuresupplied to the engaging hydraulic pressure chamber in addition to theurging forces of the first and second urging mechanisms. Note that, whenthe piston is quickly moved in the engaging direction by the urgingforces of the first and second urging mechanisms, the engaging hydraulicpressure chamber may have a negative pressure and the moving speed ofthe piston may be decreased. This trouble is also addressed by thesupply of the predetermined hydraulic pressure to the engaging hydraulicpressure chamber.

An automatic transmission according to another aspect of the presentinvention includes: a gear shifting mechanism; and the frictionalengagement element described above. The automatic transmission hasadvantages of the frictional engagement element, that is, advantages inthat the frictional engagement element is promptly switched from thereleased state to the engaged state and the engagement shock does notoccur.

According to the present invention, it is possible to provide thefrictional engagement element in which the piston can promptly be movedfrom the releasing position toward the engaging position whilepreventing the engagement shock, and to provide the automatictransmission using the frictional engagement element.

This application is based upon Japanese Patent Application No.2016-32043, filed with the JPO on Feb. 23, 2016, the contents of whichare incorporated herein by reference.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdefined, they should be construed as being included therein.

The invention claimed is:
 1. A frictional engagement element,comprising: a plurality of friction plates arranged with clearancestherebetween; a piston movable between a releasing position where thefriction plates are brought into a released state, and an engagingposition where the friction plates are brought into an engaged state bypressing the friction plates; and a first urging mechanism and a secondurging mechanism configured to urge the piston in an engaging directionfrom the releasing position toward the engaging position, wherein:urging forces of both of the first urging mechanism and the secondurging mechanism act on the piston from the releasing position to afirst position in the engaging direction; the urging force of the firsturging mechanism acts on the piston from the first position to a secondposition closer to the engaging position than the first position; thefirst position is a predetermined position where the piston does notabut against the friction plates; and the second position is a positionwhere the piston abuts against the friction plates and the clearancesare reduced.
 2. The frictional engagement element according to claim 1,wherein: the first urging mechanism is formed of a first compressioncoil spring, and the second urging mechanism is formed of a secondcompression coil spring; and a working length of the first compressioncoil spring is larger than a working length of the second compressioncoil spring.
 3. The frictional engagement element according to claim 2,wherein the urging force of the second urging mechanism is greater thanthe urging force of the first urging mechanism.
 4. The frictionalengagement element according to claim 3, wherein: the piston has adisc-like shape; and the first urging mechanism is arranged on aradially inner side of the piston, and the second urging mechanism isarranged on a radially outer side of the piston.
 5. The frictionalengagement element according to claim 4, further comprising: a releasinghydraulic pressure chamber configured to move the piston in a directiontoward the releasing position; and a hydraulic electronic controllerconfigured to control a hydraulic pressure to be supplied to thereleasing hydraulic pressure chamber, wherein when the piston is movedtoward the releasing position, the hydraulic electronic controllercauses a hydraulic pressure at least greater than the urging force ofthe first urging mechanism to be supplied to the releasing hydraulicpressure chamber.
 6. The frictional engagement element according toclaim 5, wherein when the piston is moved in the engaging direction, thehydraulic electronic controller causes the hydraulic pressure in thereleasing hydraulic pressure chamber to be discharged.
 7. An automatictransmission, comprising: a gear shifting mechanism; and the frictionalengagement element according to claim
 6. 8. The frictional engagementelement according to claim 5, further comprising an engaging hydraulicpressure chamber configured to move the piston in a direction toward theengaging position, wherein when the piston is moved in the engagingdirection, the hydraulic electronic controller causes a predeterminedhydraulic pressure to be supplied to the engaging hydraulic pressurechamber.
 9. An automatic transmission, comprising: a gear shiftingmechanism; and the frictional engagement element according to claim 8.10. The frictional engagement element according to claim 1, wherein theurging force of the second urging mechanism is greater than the urgingforce of the first urging mechanism.
 11. The frictional engagementelement according to claim 10, wherein: the piston has a disc-likeshape; and the first urging mechanism is arranged on a radially innerside of the piston, and the second urging mechanism is arranged on aradially outer side of the piston.
 12. The frictional engagement elementaccording to claim 11, further comprising: a releasing hydraulicpressure chamber configured to move the piston in a direction toward thereleasing position; and a hydraulic electronic controller configured tocontrol a hydraulic pressure to be supplied to the releasing hydraulicpressure chamber, wherein when the piston is moved toward the releasingposition, the hydraulic electronic controller causes a hydraulicpressure at least greater than the urging force of the first urgingmechanism to be supplied to the releasing hydraulic pressure chamber.13. The frictional engagement element according to claim 12, whereinwhen the piston is moved in the engaging direction, the hydraulicelectronic controller causes the hydraulic pressure in the releasinghydraulic pressure chamber to be discharged.
 14. An automatictransmission, comprising: a gear shifting mechanism; and the frictionalengagement element according to claim
 13. 15. The frictional engagementelement according to claim 12, further comprising an engaging hydraulicpressure chamber configured to move the piston in a direction toward theengaging position, wherein when the piston is moved in the engagingdirection, the hydraulic electronic controller causes a predeterminedhydraulic pressure to be supplied to the engaging hydraulic pressurechamber.
 16. An automatic transmission, comprising: a gear shiftingmechanism; and the frictional engagement element according to claim 15.17. The frictional engagement element according to claim 1, furthercomprising: a releasing hydraulic pressure chamber configured to movethe piston in a direction toward the releasing position; and a hydraulicelectronic controller configured to control a hydraulic pressure to besupplied to the releasing hydraulic pressure chamber, wherein when thepiston is moved toward the releasing position, the hydraulic electroniccontroller causes a hydraulic pressure at least greater than the urgingforce of the first urging mechanism to be supplied to the releasinghydraulic pressure chamber.
 18. The frictional engagement elementaccording to claim 17, wherein when the piston is moved in the engagingdirection, the hydraulic electronic controller causes the hydraulicpressure in the releasing hydraulic pressure chamber to be discharged.19. The frictional engagement element according to claim 17, furthercomprising an engaging hydraulic pressure chamber configured to move thepiston in a direction toward the engaging position, wherein when thepiston is moved in the engaging direction, the hydraulic electroniccontroller causes a predetermined hydraulic pressure to be supplied tothe engaging hydraulic pressure chamber.
 20. An automatic transmission,comprising: a gear shifting mechanism; and the frictional engagementelement according to claim 1.